The present invention relates to a signature log storing apparatus, and particularly to a signature log storing apparatus for providing a guarantee for validity of a digital signature.
A technology of digital signatures (hereinafter referred to as “signature”) for providing a guarantee for authenticity of an electronic document is based on a cryptography wherein the safety is guaranteed by difficulty in calculation in estimating “private key.” Accordingly, the current digital signature can guarantee the authenticity of the electronic document for a relative short period, but cannot always guarantee the authenticity of the electronic document for a long period such as 20 or 30 years. This is because there is a possibility that “private key” can be estimated by technical innovations which improve calculation speeds of computers or cryptanalysis algorisms for a long period. In addition, there is a possibility that “private key” is leaked by a human error in operation.
Once “private key” is passed on a harmful third party, this makes the authenticity of the signature generated with the “private key” cannot be guaranteed. Accordingly, an expiry date is set in a public key, and the validity of the digital signature is guaranteed up to the expiry date. Such digital signatures cannot guarantee the authenticity for electronic documents stored over a long period. To solve this problem, technologies are developed for extending the validity of the digital signatures over the expiry date of the public key certificates.
For example, D. Pinkas, J. Ross, N. Pope, “RFC3126—Electronic Signature Formats for long term electronic signatures”, IETF (Internet Engineering Task Force), September 2001, URL<http://www.faqs.org/rfcs/rfc3126.html> (hereinafter referred to as Document 1), discloses a technology for guaranteeing the validity of the digital signature over a long period as follows:
Evidence information (a certificate authority certificate, a CRL (certificate revocation List) and the like) necessary for validating a signature again is previously acquired, and a time stamp is provided to the electronic document, the signature, and the evidence information. Next, a new time stamp is issued again before expiration of the time stamp.
Further, Japanese laid-open patent application publication No. 2001-331104 (hereinafter referred to as Document 2) discloses a technology called the hysteresis signature in which a signature is left as a record upon generation of the signature, and when a new signature is generated, the record of the pervious signature is reflected in the new signature. According to this hysteresis signature technology, a document with the hysteresis signature has validity of the signature for the electronic document which is extended without re-signing for the following electronic documents as long as following documents also have the hysteresis signatures.
Further, it is said that the hysteresis signature technology can improve authenticity in a linkage relationship between signatures by opening a part of a signature log through a public medium such as periodicals such as newspapers and magazines, and websites in the Internet.